Continuous performance testing, in various forms, has become a standard clinical procedure. Generally speaking, continuous performance testing tests a subject's visual attention by displaying a series of visual stimuli, to which the subject is instructed to respond. In the typical (and simplest) case, often referred to as a “Go—No Go” test, the stimuli are of two types (the “Go” and “No Go” stimuli); the subject is instructed to respond only to the “Go” stimulus, and not to respond or “pass” when presented with the “No Go” stimulus. Data collected for each stimulus presented consists of the type of the stimulus; whether or not the subjects responded; and, if so, how long they took to respond. The continuous performance test has been in use since the 1950s (see “A continuous performance test of brain damage,” Rosvold et al. (1956), J. Consulting and Clinical Psychology 20:343-350), with computerized versions available in the 1970s and 1980s (see, e.g., “An objective measure of methylphenidate response: clinical use of the MCA,” Greenberg (1987), Psychopharmacol. Bull. 23:279-282.
While these tests provide raw data for continuous performance testing, they have typically distilled the data into a few isolated numbers, such as: Latency (the average of all response times to “Go” stimuli); Commission Errors (the number responses to “No Go” stimuli divided by the total number of “No Go” stimuli); and Accuracy (percentage correct; the correct passes to “No Go” stimuli, added together and divided by the total number of stimuli). These previous methods of analysis fail to account for a wide range of normal strategies. For example, a subject might elect to be careful, favoring accuracy over speed. Another subject could choose to be as fast as possible, willing to commit more errors in the process. Some methods do not take such strategies into account.
Alzheimer's disease (“AD”) is a degenerative brain disorder that afflicts millions of people worldwide. It is the most common form of dementia and can affect memory, mood, personality, and cognitive ability. The risk of developing AD becomes greater with age. As the average human life-span continues to increase, the number of people developing AD at some point in their lives is escalating rapidly. Currently, an estimated 1 in 20 people over the age of 65 are affected by some form of dementia. In persons over the age of 80, that number rises to 1 in 5.
The effects of AD can be devastating. Early symptoms include forgetfulness, learning difficulties, and loss of concentration. The later stages of the disease are characterized by disorientation, extreme memory loss, impairment of speech and reading comprehension, and changes in personality. Dramatic mood swings can occur, including outbursts of anger, bouts of fearfulness, and periods of deep apathy or depression. The sufferer becomes increasingly confused, particularly when confronted with unfamiliar settings, and may wander off and become lost. Physical problems, such as an odd gait, a loss of coordination, an inability to chew and swallow, and an inability to control bowel and bladder functions, gradually develop. Eventually, the patient may become totally noncommunicative, physically helpless, and incontinent. The disease is invariably fatal.
AD can also have a profound impact on the relatives of the person suffering from the disease. About seventy percent of AD patients are cared for at home by family members. In the early and middle stages of AD, patients may need help in managing their financial and business affairs. As the disease progresses, the affected person becomes steadily more dependent on caregivers to help perform daily tasks. The patient's mental functioning eventually deteriorates to the point where it is not safe to leave the person unattended. Ultimately, the disease may leave its victims bedridden and unable to care for themselves. Under these circumstances, AD can take a tremendous physical, financial, and emotional toll on the caregivers.
Although there is currently no cure for AD, early diagnosis is important for a number of reasons. For instance, it is crucial to rule out other conditions which have symptoms that are similar to AD, but which are treatable. In addition, the patient and family members can receive much help and advice from doctors and other professionals in coping with this disease. Furthermore, medications are available which can help relieve some of the common symptoms of AD, including depression, anxiety, and sleep disturbance. There is also hope that treatments may be developed in the future which will slow or halt the progression of the disease, making early detection and intervention even more vital.
Diagnosing AD can often be difficult, especially in the early stages, because many of the symptoms of the disease mirror the natural signs of aging. In some situations, a definitive diagnosis may not be possible until the patient has died and an autopsy can be performed. There are also several forms of dementia that appear superficially similar to AD, but have distinct underlying pathological processes. These dementias are often indistinguishable from AD using conventional testing techniques.
Current psychological tests for AD that are used clinically focus on deteriorations in memory, particularly in short-term or “working” memory. In general, the disorder must be fairly well advanced before significant impairments in memory are observed. Consequently, these tests are not fully capable of diagnosing AD in the early stages. Thus, there is a need for an easily administered, non-invasive, and reliable test for detecting AD while still in the early stages of development. In particular, there is a need for a reliable continuous performance test for detecting AD that takes into account a subject's rational preferences.